(1) Field of the Invention
The present invention relates to a heat-resistant multi-layer drawn polyester vessel. More particularly, the present invention relates to a process in which a multi-layer drawn polyester vessel comprising a layer of a thermoplastic polyester such as polyethylene terephthalate and a gas-barrier resin layer is efficiently prepared by using only one mole, and also to a drawn polyester vessel excellent in heat resistance, resistance to thermal contraction and resistance to heat distortion, which is prepared according to this process.
(2) Description of the Prior Art
A drawn bottle composed of polyethylene terephthalate (PET) is excellent in transparency, impact resistance (strength against falling shock), light weight, sanitary characteristics, gas barrier properties to oxygen, carbon dioxide gas and the like and pressure resistance, and this bottle is widely used as a packaging vessel for soy sauce, sauce, dressing, edible oil, carbonated drinks such as beer, cola and soda pop, fruit juice, mineral water, shampoo, detergent, cosmetics, wine, mustard and aerosol.
In case of a glass bottle or a metal can in which sealing is complete, the gas permeability is regarded as being substantially zero, but a drawn polyester vessel which is a kind of a plastic vessel has a permeability to oxygen, carbon dioxide gas or the like though it is very slight. Accordingly, the drawn polyester vessel is inferior to a metal can or glass bottle in the content-preserving property, and in case of a carbonated drink, carbon dioxide gas is lost and in case of beer, cola or soda pop, the storage period is definitely limited. Furthermore, in a juice-containing liquid, the storage period is similarly limited because of permeation of oxygen from the outside.
As means for eliminating this defect, there has been proposed a vessel having a multi-layer structure comprising inner and outer layers of a polyester and an intermediate layer of a gas-barrier resin such as an ethylene/vinyl alcohol copolymer or a metaxylylene group-containing polyamide.
The drawn polyester vessel is much superior to other plastic bottles in transparency and pressure resistance in case of a gas-incorporated drink, but the draw-forming temperature of the drawn polyester vessel is relatively low (80.degree. to 110.degree. C.) and there is present an undrawn portion or a lowly drawn portion. Accordingly, the heat resistance is poor and in case of hot packing, a practically applicable product cannot be obtained unless the filling temperature is lower than 65.degree. C., and the shape-retaining property is lost if hot packing is carried out at a higher temperature.
As means for eliminating this defect, there has been proposed a method in which an undrawn portion of a polyester bottle (such as a mouth-neck portion) and a drawn portion (such as a barrel portion) are heat-treated (heat-set). As this heat treatment method, there are known a so-called one-mole method in which a mold for draw-blow forming is maintained at a high temperature for the heat treatment, after draw-blow forming the heat treatment is carried out in this mold and the product is cooled in the mold and taken out from the mold (see, for example, Japanese patent publication No. 6216/84) and a so-called two-mold method in which a mold for draw-blow forming is maintained at a high temperature for the heat treatment, after draw-blow forming the heat treatment is carried out in the mold, the draw-blow-formed product is taken out and put in a cooling mold and the product is cooled in the secondary mold under inflation (see, for example, Japanese patent application Laid-Open Specification No. 53326/82).
A conventional single-layer PET bottle has appropriate gas barrier properties, but in view of the technical level at the present, this PET bottle is still insufficient in the kind of the content and the transportation form. As means for improving the gas barrier properties of a polyester, there has been proposed a method in which a multi-layer structure is formed. However, this method is not practically carried out. Moreover, a method for improving the heat resistance and the resistance to heat distortion while maintaining excellent gas barrier properties has not been proposed.
In the case where the heat treatment is carried out at the draw-blowing step, the formed body is cooled by cooling the blow mold by passing cooling water in the mold, and the temperature of the formed body is lowered to a temperature at which withdrawal of the formed body from the mold is possible. Then, the formed body is taken out from the mold, and an intended heat-treated polyester bottle is obtained.
However, in the former one-mold heat treatment method, since the heat treatment and the interior cooling treatment are carried out in one blow mold and a certain time is necessary for each treatment, the residence time in one mold becomes long and the forming time is 2 to 4 times as long as the forming time in ordinary draw-forming of a polyester bottle, resulting in drastic reduction of the production efficiency and increase of the manufacturing cost. Moreover, if the heat treatment temperature is elevated, a long time is necessary for lowering the temperature to a level at which the formed body can be taken out from the mold. Accordingly, the heat treatment is naturally lowered, and the obtained bottle is inferior in the heat resistance and the resistance to thermal contraction.
In the latter two-mold heat treatment method, two kinds of molds and two-staged blowing operation are necessary and the step number is increased, resulting in increase of the equipment cost. In order that the primarily blow-formed body taken out after the heat treatment is put in a secondary mold assuredly, the size of the primarily blow-formed body should be smaller than the size of the final formed body while the safety rate is taken into consideration. Accordingly, certain expansion drawing should be performed to some extent at the secondary blowing step and thermal contraction corresponding to this expansion drawing at the second stage of blowing is readily caused. Moreover, the two-mold method is defective in that it is difficult to faithfully reproduce the surface configuration of the mold cavity on the surface of the blow-formed body.